The use of multi-cables to lift loads, either due to the increasing weight of the load, or increasing operating depth, or both, is known. Loads of hundreds of tonnes are no longer exceptional, typically often now over 400 tonnes.
Sometimes, multi-cable lifting systems are formed using a single looped cable as shown in EP0312336A. Sometimes, the equipment involves a number of separate cables, ropes, wires, etc., as shown for example in WO03/062042A1, U.S. Pat. No. 5,951,227 and WO2010/046649A1. Generally, the cables are provided from a reel, and optionally run through one or more drive mechanisms or winches to control their movement and loading.
“Cabling” is a phenomenon whereby imbalanced torsional characteristics of two cables, being used in 2-fall or parallel operation to lower or raise a load, can result in the cables axially rotating, causing the effective cable separation to decrease, and possibly resulting in rotational entanglement of the two cables employed. This can occur both in single cable hoist systems used in 2-fall configuration, as well as in parallel winch operations using cables of opposite hand lay.
Naturally, cabling is undesirable. It can even lead to situations where the integrity of the hoist cables is compromised, with damage resulting, or even overloading of a single wire. The required recovery of the cables can then be difficult, involving extensive OPEX including delay.
Meanwhile, there is an ever increasing drive to work at greater water depths, certainly beyond 1000 meters, and now commonly in excess of 2000 meters or even 3000 meters. The greater the required depth, the greater the extension of cables from their lifting platform or base, and the greater potential for cabling. Cabling at greater depths requires even greater OPEX.
WO2012/060715A1 shows a lifting tool for opposing twisting of generally submerged ropes, the lifting tool comprising a body with a centre axis and having a operable lock adapted to catch a rope connector, and a structure that is designed to be connected to a hoist or crane, wherein the lifting tool is equipped with at least one water flow inducing means positioned at a radial distance from the centre axis. Preferably, this includes rudders. WO2012/060715A1 is particularly concerned with hoisting operations at sea with heavy items, and where the hoisting operations are heave-compensated, such that the lifting rope will be continuously reeled in and out from a winch due to the heave motion of the lifting vessel. As it states, “an inherent problem when utilising parallel ropes is the tendency to twist and to get entangled in each other. As the ropes have to move independently of each other in the sea, an entanglement may in a worst case lead to cutting of the ropes and the loss of a valuable item.”